Two-fold bellows and a method of making the same have been known for a long time and reference can be made to U.S. Pat. Nos. 2,221,470; 2,814,072 and 4,749,345.
A vulcanization mold for manufacturing two-fold bellows usually comprises three segments, namely two end segments and a center segment. These three segments are, at first, spaced from each other, and are brought together in the course of the vulcanization process. The center segment remains at rest. Only the two outer segments are moved synchronously toward the center segment.
For making three-fold bellows, one requires two inner segments between the two end segments. A center segment at rest is non-existent. A synchronous moving together of the four segments is not easily possible because the individual segments have to be moved at different speeds.